1. Field of the Invention
The present invention relates to an isocyanate-based polymer foam and to a process for production thereof. More particularly, the present invention relates to an isocyanate-based polymer foam, inter alia, having improved properties over a broader temperature range compared to prior art foams.
2. Description of the Prior Art
Isocyanate-based foams, such as polyurethane foams, are known in the art.
It is known in the art that polyurethane foams have energy dissipating properties. Thus, heretofore, such foams have been used in helmets, shoe insoles, furniture, seating applications and the like. These foams have also found widespread use in vehicular applications such as head rests, arm rests, door panels, knee bolsters, air bag doors, headliners, bumpers, instrument panels, sun visors and other areas of the vehicle intended to dissipate energy upon impact.
One class of polyurethane foams which is of interest is the so-called xe2x80x9clow resiliencexe2x80x9d, flexible polyurethane foams. Such foams have a low rate of recovery from an applied stress and a low resilience as measured by the ball rebound test. Specifically, low resilience, flexible polyurethane foams are known to have ball rebound values of less than about 30% (cf. about 40% for conventional slabstock polyurethane foam and 55-60% for high resilience, flexible polyurethane foams).
One problem with known low resilience, flexible polyurethane foams is that they are very temperature sensitive. Specifically, such foams will soften with increasing temperature and harden with decreasing temperature. The low resiliency properties of these foams is typically lost as the temperature increases. Further, it is known that other physical properties of these foams are quite sensitive to a number of factors including: (i) variations in isocyanate index of the formulation used to produce the foams, (ii) relative humidity, and the like.
The temperature sensitivity of these known foams has been used to advantage in certain applications. For example, since body heat can be sufficient to cause softening and conformation to shape of area being contacted, such foams have been used successfully in applications such as headphones, ski boots, hiking boots, skates and the like.
There are, however, a number of applications where it would be desirable to have a foam which maintains its properties over a relatively wide temperature rangexe2x80x94i.e., a foam which was less temperature sensitive than currently known low resilience, flexible polyurethane foams. Such a foam would find many applications such as for use in the interior of a vehicle where energy dissipation of the foam is important over a relatively wide temperature rangexe2x80x94e.g., xe2x88x9220xc2x0 to +80xc2x0 C. As will be appreciated by those of skill in the art, the use of a foam in the interior of a vehicle presents specific challenges since the service temperature of the vehicle may vary within such broad temperature ranges. Thus, a foam which is designed to dissipate energy upon impact and which behaves differently at different temperatures is disadvantageous.
Accordingly, it would be desirable to have an isocyanate-based foam, preferably a polyurethane foam which is a low resilient foam and which has an energy dissipation profile substantially unaffected over a relatively wide temperature range.
It is an object of the present invention to provide a novel isocyanate based foam which obviates or mitigates at least one of the above-mentioned disadvantages of the prior art.
It is another object of the present invention to provide a novel process for producing an isocyanate-based foam.
Accordingly, in one of its aspects, the present invention provides a flexible isocyanate-based polymeric foam which has:
(i) low resiliency;
(ii) a Tg less than or equal to about 0xc2x0 C.; and
(iii) a change in tan xcex4 less than or equal to about 35% from a median value measured over a temperature range of from about xe2x88x9220xc2x0 to about +60xc2x0 C.
In another of its aspects, the present invention provides a flexible, low resiliency foam derived from a reaction mixture comprising:
urethane-forming chemicals;
water; and
a plasticizer selected from the group comprising: a halogenated paraffin, a C2/C4 aliphatic polymer comprising a primary hydroxyl group, and mixtures thereof.
As is known in the art, viscoelastic materials are those for which the relation between stress and strain depends on time. The stiffness of such materials will depend on the rate of load applicationxe2x80x94e.g., increased rates increase the stiffness. Viscoelasticity is a characteristic of long chain structures (the difference between crystalline solids (metals) and plastics). As stated above, viscoelasticity is a time dependent property (e.g., delayed response), rendering polymers elastic solids and viscous liquids simultaneously. Under a dynamic load, a phase difference appears between stress and strain.
Thus, the present inventors have surprisingly and unexpected discovered that a particular selection of additives used to produce the isocyanate-based foam results in a foam having significantly improved properties. More particularly, the use of specific plasticizers, as set out herein, results in a low resiliency isocyanate-based foam having a glass transition temperature Tg less than or equal to about xe2x88x920xc2x0 C. and a change in tan xcex4 less than or equal to about 35% from a median value measured over a relatively broad temperature range. A foam having such a combination of properties is highly desirable, and to the knowledge of the present inventors, is heretofore unknown. While not wishing to be bound by any particular theory or mode of action it is believe that the advantages of the present isocyanate-based foams may be the result of mechanical and/or chemical activity of the specific plasticizers set out herein which complements the properties of the foam based on the chemical structure design.
Preferably, Tg and tan xcex4 are measured using the equipment and testing protocols set in the Examples hereinbelow.
Glass transition temperature or xe2x80x9cTgxe2x80x9d, as used throughout this specification, is intended to mean the temperature (in some cases, this may be a range of temperatures) when a polymer material softens with some polymer segments movingxe2x80x94maximum energy dissipation is achieved by the polymer at this point. Above the Tg, the polymer tends to be rubbery and, as the temperature is continuously increased, the polymer with melt
The present isocyanate-based foams will find immediate advantageous use in a number of applications. For example, the present isocyanate-based foams may be used in vehicular applications such as head rests, arm rests, door panels, knee bolsters, air bag doors, headliners, bumpers, instrument panels, sun visors and other areas of the vehicle intended to dissipate energy upon impact. The present isocyanate-based foams are particularly useful in vehicular head rests.